Diesel particulate filter control

ABSTRACT

Methods and systems are provided for managing particulate emissions in an engine including a particulate filter and a CO 2  sensor downstream of the filter. A CO 2  sensor may be used to infer the presence of particulate matter in the exhaust, downstream of the filter. By sensing particulate matter in the post-filter exhaust, filter degradation may be identified.

FIELD

The present description relates to particulate matter purifying systemsfor internal combustion engines, such as diesel engines and gasolineengines.

BACKGROUND AND SUMMARY

Emission control devices, such as diesel particulate filters (DPF), mayreduce the amount of particulate matter emissions (such as, soot) from adiesel engine by trapping the particles. Such devices may be regeneratedduring engine operation to decrease the amount of trapped particulatematter (for example, by burning) and maintain the collection capacity ofthe device. To meet stringent federal government emissions standards,regeneration operations and DPF functionality may be tightly controlledand regularly assessed.

One example approach for controlling diesel particulate filters isillustrated by Stewart et al. in U.S. Pat. No. 7,155,334. Therein, anengine controller initiates filter regeneration based on inputs receivedfrom sensors, such as particulate matter sensors and/or carbon dioxidesensors, positioned upstream and downstream of the filter. Specifically,the controller determines whether to regenerate the filter based on thecomposition of exhaust gas before and after passage through the filter.

However, the inventor herein has recognized issues with such anapproach. As one example, the use of resistive sensing-based particulatematter (PM) sensors reduces the sensitivity of the emission controlsystem. As such, commonly used PM sensors may be configured to detectthe presence of PMs electrically, based on a change in resistance orcapacitance across an electrical circuit. Such sensors may have a“dead-band” during which PMs may have to accumulate before the sensor isable to respond. This additional time required to detect PMs may delaythe determination and initiation of filter regeneration. As such, thedelay in filter regeneration may accelerate filter degradation. Theadditional time may also reduce the electrical sensor's ability toidentify DPF degradation. Overall, this may lead to degraded exhaustemission levels.

As another example, the use of input from CO₂ sensors that sense exhaustCO₂ levels in determining filter regeneration may reduce the system'sability to accurately estimate the soot load on the filter due to anindirect correlation between filter soot levels and exhaust CO₂ levels.Since the exhaust CO₂ level is more representative of combustionconditions, a soot load may be inferred but not accurately determined.

Thus, in one example, some of the above issues may be addressed by amethod of operating an engine exhaust system including a particulatefilter comprising adjusting engine operation based on a CO₂ signature ofoxidized, post-filter exhaust particulate matters (PMs). The CO₂signature may include a CO₂ level of oxidized PMs estimated by a CO₂sensor positioned downstream of the filter.

In one example, a diesel engine exhaust system may be configured with afilter substrate and a CO₂ sensor positioned downstream of a DPF. Duringselected engine running conditions, an engine controller may heat thesubstrate and oxidize post-filter exhaust particulate matters (that is,exhaust soot) on the heated substrate using oxygen present in theexhaust gas. The CO₂ generated from the oxidation of the soot may beestimated by the downstream CO₂ sensor to determine a CO₂ signature ofthe oxidized, post-filter exhaust particulate matters (PMs). The CO₂signature may at least include a CO₂ level of the oxidized PMs. Sincethe generated CO₂ is largely dependent on the quantity of exhaust sootoxidized on the heated substrate, a direct correlation may be madebetween the estimated exhaust CO₂ level and an exhaust soot level. Inother words, the CO₂ sensor may be used as a PM sensor. The controllermay then adjust engine operation and perform filter diagnostics based onthe CO₂ signature. During other engine running conditions, the CO₂sensor may be used to sense an exhaust CO₂ level unrelated topost-filter exhaust PMs.

For example, during a first engine running condition, when the filter isstoring and the substrate is not oxidizing (e.g., a substrate heater isturned off), the engine controller may adjust engine operations (suchas, EGR operations) based on an exhaust CO₂ level estimated by the CO₂sensor. During a second engine running condition, different from thefirst engine running condition, when the filter is storing and thesubstrate is oxidizing (e.g., a substrate heater is turned on), theengine controller may adjust engine operations (such as, initiation offilter regeneration) and perform filter diagnostics (such as, determinethat a filter is leaking) based on the estimated CO₂ level of oxidized,post-filter exhaust PMs. As such, during such conditions, low tosubstantially no exhaust PMs may be expected in the post-filter exhaust.Herein, by comparing the output of the CO₂ sensor to CO₂ levels expectedbased on the engine's operating conditions, exhaust PMs may beidentified in the post-filter exhaust and may be used to infer filterdegradation. For example, the CO₂ sensor may sense post-filter exhaustCO₂ levels in real-time to provide a real-time indication of thepresence of PMs in the exhaust. In response to the estimated CO₂ level(that is, sensor output) being greater than the expected CO₂ level, anengine controller may indicate filter degradation due to the presence ofPMs in the exhaust. In the event of filter degradation, the controllermay further adjust engine operating conditions, and regenerationconditions, in response to the indication of degradation. In comparison,if the estimated level is not greater than the expected level, but isgreater than a threshold, the controller may infer that the filter isnot degraded, but that sufficient soot has accumulated on the upstreamfilter, and that a filter regeneration operation should be initiated.Accordingly, filter regeneration may be initiated to restore thefilter's collection capacity.

It will be appreciated that while the depicted example illustratesapplication of the CO₂ sensor in a diesel engine exhaust system, this isnot meant to be limiting, and the same CO₂ sensor may be similarlyapplied in alternate engine exhaust systems, such as to diagnose agasoline particulate filter in a gasoline engine exhaust system.

In this way, the presence of soot in engine exhaust may be detected byoxidizing the soot to generate CO₂, and by using downstream CO₂ sensorsto provide a more direct and more precise estimate of exhaust sootlevels, in addition to their use in estimating exhaust CO₂ levels. Byenabling an accurate, real-time estimate of exhaust soot levels, filterregeneration may be better determined and more accurately initiated.Additionally, the higher sensitivity of the CO₂ gas sensors may reducethe “dead-band” effect of resistive sensors and provide higherresolution between signals. This higher resolution may improve theability to identify a degraded particulate filter. By improving accuracyin filter regeneration and filter diagnostics, the quality of exhaustemissions may be improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of an engine and an associatedparticulate matter retaining system.

FIGS. 2-5 show high level flow charts illustrating routines that may beimplemented for controlling and diagnosing filter regeneration andfilter functionality, using CO₂ sensors, in the particulate matterretaining system of FIG. 1, according to the present disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for controllinga particulate matter retaining system, such as the diesel particulatematter (PM) retaining system of FIG. 1. As shown therein, the PMretaining system may include a diesel particulate filter, a filtersubstrate, and a CO₂ gas sensor positioned downstream of the filter andthe substrate. As shown in FIG. 2, when the substrate is not heated,that is, the substrate is not enabled for oxidizing; the CO₂ sensor maybe used to estimate exhaust CO₂ levels and adjust engine operation basedon the estimated value. When the substrate is enabled for oxidizing (forexample, when the substrate is electrically heated by a dedicatedsubstrate heater, or non-electrically heated by hot exhaust gas),post-filter exhaust may be passed over the heated substrate and exhaustPMs, if present, may be oxidized to CO₂ using oxygen from the exhaustgas on the substrate upstream of the CO₂ sensor. Herein, the CO₂ sensormay be used to more accurately infer the presence of exhaust PMs, and anexhaust PM level, due to a direct correlation between the CO₂ gasgenerated on the substrate and the oxidized PMs. Accordingly, a CO₂signature may be generated by the sensor during filter storing and/orregeneration conditions. As shown in FIGS. 3-5, by comparing anestimated CO₂ level (based on the sensor output) with an expected CO₂level (based on, for example, engine operating or regeneration operationconditions), filter degradation may be identified, and distinguishedfrom a degraded regeneration. In this way, a CO₂ sensor may beadvantageously used to accurately identify the presence of PMs inpost-filter exhaust, and provide more accurate control over filterfunctionality and regeneration efficacy. By reducing the need forresistive-sensing PM sensors, higher signal resolutions may be achieved,enabling improved emissions control.

FIG. 1 shows a schematic depiction of a vehicle system 6. The vehiclesystem 6 includes an engine system 8 coupled to a particulate matter(PM) retaining system 22. The engine system 8 may include an engine 10having a plurality of cylinders 30. Engine 10 includes an engine intake23 and an engine exhaust 25. Engine intake 23 includes a throttle 62fluidly coupled to the engine intake manifold 44 via an intake passage42. The engine exhaust 25 includes an exhaust manifold 48 eventuallyleading to an exhaust passage 35 that routes exhaust gas to theatmosphere. Throttle 62 may be located in intake passage 42 downstreamof a boosting device, such as a turbocharger, (not shown) and upstreamof an after-cooler (not shown). When included, the after-cooler may beconfigured to reduce the temperature of intake air compressed by theboosting device.

Engine exhaust 25 may include one or more emission control devices 70,which may be mounted in a close-coupled position in the exhaust. One ormore emission control devices may include a three-way catalyst, lean NOxfilter, SCR catalyst, etc. Engine exhaust 25 may also include PMretaining system 22 positioned upstream of emission control device 70.In one example, as depicted, PM retaining system 22 is a dieselparticulate matter retaining system. Tailpipe exhaust gas that has beenfiltered of PMs, following passage through PM retaining system 22, maybe further processed in emission control device 70 and expelled to theatmosphere via exhaust passage 35.

PM retaining system 22 may include one or more PM retaining devices,such as a diesel particulate filter (DPF) 102, to temporarily filter PMsfrom entering gases. DPF 102 may have a monolith structure made of, forexample, cordierite or silicon carbide, with a plurality of channelsinside for filtering particulate matter from diesel exhaust gas. PMretaining system 22 may further include substrate 104 positioneddownstream of DPF 102, and a CO₂ gas sensor 106 positioned downstream ofsubstrate 104. Substrate 104 may be heated non-electrically, for exampleby the passage of heated exhaust gas, or electrically, for example bythe operation of a dedicated substrate heater 108, to oxidizepost-filter exhaust PMs, or soot, into CO₂. A CO₂ signature of thepost-filter oxidized PMs may then be obtained using the downstream CO₂sensor.

Substrate 104 may have a monolith structure similar to DPF 102, made of,for example, cordierite, silicon carbide, etc. The substrate may not becatalyzed, consequently functioning similar to a filter. In one example,the diameter of substrate 104 may be configured to match the diameter ofexhaust passage 35 so as to enable all exhaust gas to glow through thesubstrate. The length of substrate 104 may be adjusted based on desiredstorage properties. In the depicted embodiment, wherein substrate 104may not have substantial soot storing functions, a shorter substrate maybe used (for example, 50 mm or less). In alternate embodiments, whereinsubstrate 104 may be required to store soot in addition to oxidizingexhaust soot, a longer substrate may be employed.

Substrate 104 may be heated by substrate heater 108. In one example, asdepicted, heater 108 may be an internal heater, such as an electricalresistance heater woven into the substrate. The electrical resistanceheater may be controlled, using PWM control, to adjust the temperatureof the substrate to a desired temperature. In alternate examples, theheater may be an external heater. In one example, the operation ofsubstrate heater 108 may be based on exhaust gas temperature. Thus, thesubstrate heater may be turned on when the exhaust gas temperature isbelow a threshold (such as during filter storing), and turned off whenthe exhaust gas temperature is above the threshold (such as duringfilter regeneration). Alternatively, substrate heater operation may bebased on filter conditions. Herein, the heater may be turned on when thefilter is in a storing mode, and may be turned off when the filter is ina regenerating mode.

While the depicted example illustrates substrate 104 downstream of DPF102, in alternate embodiments, substrate 104 may be included in DPF 102.During engine operation, substrate 104 may be heated, for example, dueto the flow of heated exhaust gas over the substrate. Alternatively,substrate 104 may be heated by a dedicated heater. Heated substrate 104may oxidize exhaust PMs on the substrate using oxygen from the exhaust.The oxidized PMs, such as soot, may thereby be combusted to generateCO₂. Downstream CO₂ sensor 106 may be configured to provide an accurateestimate of the generated CO₂ level. An exhaust PM level may then bedetermined or inferred by an engine controller 12 based on the estimatedCO₂ level.

During engine operation, exhaust gas PMs may be retained and collectedon DPF 102. Thus, an amount of post-filter exhaust PMs may besubstantially low in the absence of filter degradation. Consequently,there may be substantially no difference between a CO₂ level sensed bythe CO₂ sensor (that is, a sensor output) and a CO₂ level expected basedon engine operating conditions. In the event of filter degradation (suchas, due to a crack in the filter), soot may escape from the filter andmay be detected due to a sudden increase in CO₂ levels estimated by thedownstream sensor, due to oxidation of the released soot on thepost-filter substrate. Post-filter exhaust PM levels may be determinedbased on CO₂ levels sensed by CO₂ sensor 106, downstream of substrate104. For example, a real-time estimate of PM levels may be obtained. Asfurther elaborated with reference to FIGS. 2-4, by monitoring theestimated CO₂ levels and/or inferred PM levels, DPF 102 operations maybe assessed and filter diagnostic routines may be performed. In oneexample, in response to an increase in CO₂ levels above a threshold,during engine operation, filter saturation may be deduced and filterregeneration may be initiated. In another example, in response to CO₂levels being above an upper threshold (or below a lower threshold)during and/or following filter regeneration, filter degradation may bediagnosed and filter diagnostic codes may be set.

Vehicle system 6 may further include control system 14. Control system14 is shown receiving information from a plurality of sensors 16(various examples of which are described herein) and sending controlsignals to a plurality of actuators 81 (various examples of which aredescribed herein). As one example, sensors 16 may include exhaust gassensor 126 (located in exhaust manifold 48), temperature sensor 128 andpressure sensor 129 (located downstream of emission control device 70),and CO₂ sensor 106 (located downstream of DPF 102 and substrate 104).Other sensors such as additional pressure, temperature, air/fuel ratio,and composition sensors may be coupled to various locations in thevehicle system 6. As another example, the actuators may include fuelinjectors 66, throttle 62, DPF valves that control filter regeneration(not shown), etc. The control system 14 may include a controller 12. Thecontroller may receive input data from the various sensors, process theinput data, and trigger the actuators in response to the processed inputdata based on instruction or code programmed therein corresponding toone or more routines. Example control routines are described herein withreference to FIGS. 2-5.

FIG. 2 illustrates an example routine 200 for adjusting engine andfilter operation based on feedback from a CO₂ sensor positioneddownstream of a particulate filter in an engine exhaust. Specifically,by using the CO₂ sensor to estimate an exhaust CO₂ level and/or inferthe presence of post-filter exhaust PMs, filter diagnostics may beperformed during all engine running conditions including filter storingand filter regeneration.

At 202, the routine includes confirming that the engine is running. Ifthe engine is not running, the routine may end. At 204, engine operatingconditions may be estimated and/or inferred. These may include, forexample, driver requested torque, engine temperature, exhaust gastemperature, exhaust gas composition, air-fuel ratio, etc. At 206, itmay be determined whether a substrate positioned downstream of theparticulate filter in the particulate matter retaining system (FIG. 1)is oxidizing. As such, the substrate may be enabled for oxidizing PMswhen it is heated (for example, above a threshold temperature). In oneexample, the substrate may be electrically heated by operating thededicated substrate heater. In another example, the substrate may benon-electrically heated by the flow of hot exhaust gas there-through. Assuch, when the substrate is hot and enabled for oxidizing, exhaust PMs,if present, may be collected and oxidized on the substrate using oxygenfrom the exhaust. Thus, if the substrate is not oxidizing, at 208, theCO₂ sensor may provide an estimate of exhaust CO₂ levels only, and anengine controller may be configured to adjust engine operation andperform engine component diagnostics based on the estimated exhaust CO₂level. For example, the exhaust CO₂ level may be used to adjust anamount of EGR, an amount of boost, valve timing, etc. Similarly, theexhaust CO₂ level may be used to diagnose degradation in components suchas EGR components, etc.

If the substrate is oxidizing, at 209, it may be determined whetherfilter regeneration has been enabled. If filter regeneration has notbeen enabled, then at 210, expected CO₂ levels (CO2_model_nonregen) maybe estimated based on engine operating conditions. At 212, the estimatedCO₂ level may be compared to a CO₂ signature output by the sensor(CO2_sensor_non-regen). At 214, filter diagnostics may be performed andengine operation may be adjusted based on the comparison between theestimated CO₂ level and the CO₂ signature of the sensor. Herein, the CO₂sensor output may reflect a CO₂ signature corresponding to exhaust CO₂levels including CO₂ generated by the oxidation of exhaust PMs on theheated substrate. Thus, the CO₂ signature may be used to infer thepresence of post-filter exhaust PMs, and a post-filter exhaust PM level.In one example, inferring an exhaust PM level based on the estimated CO₂signature may include determining what fraction of the CO₂ signaturecorresponds to non-soot exhaust CO₂ levels, and subtracting thatfraction from the estimated CO₂ signature. Alternate algorithms may beused to compensate for exhaust non-soot CO₂ levels when determining theCO₂ level corresponding to the oxidized post-filter exhaust PMs. Asfurther elaborated with reference to FIG. 3, adjusting engine operationsand performing filter diagnostics based on a comparison between theestimated CO₂ level and the sensor output CO₂ signature may includediagnosing leak of exhaust PMs from a degraded filter, initiating filterregeneration and/or adjusting filter regeneration conditions (exhaustflow rate, exhaust temperature, etc.) based on the CO₂ signature.

Thus, during a first engine running condition, when the filter isstoring and the substrate is not oxidizing, engine operation may beadjusted based on an exhaust CO₂ level estimated by the CO₂ sensor.Then, during a second engine running condition, different from the firstengine running condition, when the filter is storing and the substrateis oxidizing (e.g., being heated by a substrate heater), engineoperation may be adjusted based on the estimated CO₂ level of oxidized,post-filter exhaust PMs.

Now turning to FIG. 3, an example routine 300 is shown for diagnosingfilter degradation and adjusting engine operation during engine runningconditions where the substrate is oxidizing and the filter is storing.That is, the depicted routine may be performed during non-regenerationconditions. As such, during filter storing conditions, exhaust PMs maybe retained in the filter and substantially no PMs may be present in thepost-filter exhaust. Consequently, a degraded filter may be diagnoseddue to the presence of PMs in the post-filter exhaust, based on a CO₂signature determined by an exhaust CO₂ sensor downstream of theoxidizing substrate. By using a CO₂ sensor, a degraded filter may bedistinguished from a marginal filter, and filter regeneration may beinitiated accordingly.

At 302, the routine may include determining an expected CO₂ level(CO2_model_nonregen) based on the estimated engine operating conditions.For example, an expected CO₂ level may be based on an expected sootload, an exhaust air-fuel ratio, a duration of engine operation sinceengine start, an exhaust flow rate, etc. At 304, post-filter exhaust maybe oxidized on the heated substrate. If present, exhaust PMs that havenot been retained in the filter may be oxidized on the heated substrateusing oxygen from the exhaust gas. In one example, during filterstoring, when the exhaust gas temperature is below a threshold, thesubstrate may be electrically heated by operating a substrate heater. At306, the CO₂ sensor's output may be determined (CO2_sensor_nonregen),and a CO₂ signature may be determined. The CO₂ signature may account forCO₂ from non-PM CO₂ sources, such as exhaust CO₂ from in-cylindercombustion events, as well as CO₂ generated due to the presence of PMsin the post-filter exhaust.

At 308, it may be determined whether the estimated CO₂ level as sensedby the CO₂ sensor (at 306) is higher than the expected CO₂ level (asdetermined based on models at 302). If yes, then at 310, it may bedetermined that soot (that is, exhaust PMs) are present in the exhaust.As previously elaborated, in the absence of filter degradation, exhaustPMs are retained in the filter. Thus, at 312, in response to thedetection of post-filter exhaust PMs, filter degradation may bedetermined and indicated by setting a diagnostic code. Setting adiagnostic code may include, for example, illuminating a light on avehicle dashboard to indicate filter degradation to the vehicleoperator. Optionally, based on the CO₂ signature, a degree of filterdegradation may also be indicated. For example, a smaller differencebetween the estimated level output by the sensor and the expected levelmay reflect a smaller degree of filter degradation (such as, release ofPMs due to a small filter leak), while a larger difference may reflect alarger degree of filter degradation (such as, release of PMs due to alarge filter crack). At 314, based on the indication of degradation,filter regeneration and engine operating conditions may be adjusted. Inone example, in response to the indication of degradation, a subsequentfilter regeneration operation may be performed for a shorter duration,at a relatively lower temperature, and/or in response to a lowerthreshold of stored PMs. In another example, in response to theindication of filter degradation, an engine load may be reduced toreduce the amount of PMs generated.

In comparison, if the estimated exhaust CO₂ level is below the expectedlevel, then at 316, it may be determined that no soot is present in theexhaust. Consequently, at 318, it may be inferred that the filter is notdegraded and can retain exhaust PMs. At 320, it may be furtherdetermined whether the estimated exhaust CO₂ level is above a threshold.If the estimated exhaust CO₂ level is contained below the threshold,then at 324, it may be determined that the filter has not yet reachedits storage capacity and filter regeneration is not required. If theestimated exhaust CO₂ level is above the threshold, then at 322 it maybe determined that the filter has approached its storage capacity andfilter regeneration may be initiated to restore the filter's storagecapacity.

Now turning to FIG. 4, an example routine 400 is described fordiagnosing filter degradation during a filter regeneration operation.Additionally, the routine may enable the filter regeneration operationto be monitored, and allow a degraded filter regeneration operation tobe distinguished from a degraded filter condition.

At 402, the substrate heater may be disabled (e.g., turned off) andfilter regeneration may be initiated. As such, filter regeneration maybe initiated based on engine operating conditions such as, enginetorque, exhaust gas temperature, flow rate, and composition, etc. In oneexample, filter regeneration may be initiated responsive to the exhausttemperature being above a threshold, a threshold duration since theengine start having elapsed, a threshold duration since a previousfilter regeneration operation having elapsed, etc.

As such, during filter regeneration, hot exhaust gas may be directed ata predetermined flow rate through the filter to burn particulate matterstored therein during a previous storing cycle. Thus, duringregeneration, the substrate heater may not be operated while the heatedexhaust is advantageously used to also heat the downstream substrate. Inone example, the heater may be controlled responsive to exhausttemperature. For example, when the exhaust gas temperature is above athreshold (such as above a regeneration temperature), the substrateheater may be turned off, and when the exhaust gas temperature is belowthe threshold, the substrate heater may be turned on.

At 404, the regeneration conditions may be estimated. These may includeestimating an exhaust gas flow rate and temperature to be used for theregeneration. Additionally, the soot load on the filter beforeregeneration may be estimated. At 406, based on the estimatedregeneration conditions, a corresponding expected CO₂ regenerationsignature/CO₂ level may be determined (CO2_model_regen). In one example,the expected CO₂ signature may be a CO₂ level profile expected for theduration of the regeneration. As such, the CO₂ level may increase ordecrease depending on conditions during filter regeneration. In oneexample, the CO₂ level may be expected to increase soon after theregeneration is initiated as the stored PMs are burned off, and then theCO₂ level may decrease. At 408, post-filter exhaust PMs may be oxidizedon the heated substrate using oxygen from the exhaust gas. At 410, a CO₂level of the post-filter exhaust (CO2_sensor_regen) may be estimated bythe downstream CO₂ sensor. Accordingly, an estimated CO₂ signature maybe determined. The estimated CO₂ level may reflect CO₂ from non-PMsources, such as CO₂ from in-cylinder combustion events, as well as CO₂released from the burning of PMs stored in the filter. Additionally, theestimated CO₂ level may reflect CO₂ from post-filter exhaust PMs, ifpresent, that are oxidized on the heated substrate. At 412, the sensoroutput estimated regeneration CO₂ level may be compared to the modelbased expected regeneration CO₂ level and it may be determined if thesensor output is higher than the modeled value. If yes, then at 414, itmay be determined that soot is present in the exhaust. As previouslyelaborated, in the absence of filter degradation, exhaust PMs areretained in the filter. Thus, at 416, in response to the detection ofpost-filter exhaust PMs, filter degradation may be determined andindicated by setting a diagnostic code. Optionally, as previouslyelaborated in FIG. 3 (at 312), based on the CO₂ signature, a degree offilter degradation may also be indicated. At 418, based on theindication of degradation, engine operating conditions and/orregeneration conditions for the same and/or a subsequent regenerationoperation, may be adjusted. In one example, in response to theindication of degradation, the filter regeneration operation may bestopped (for example, immediately) to reduce the risk of degradedexhaust emissions. In another example, a regeneration exhaust flow ratemay be decreased, a regeneration exhaust temperature may be lowered, aduration of regeneration may be decreased, and/or regeneration may beinitiated at a higher soot load. In another example, in response to theindication of filter degradation, the engine load may be reduced toreduce the amount of PMs generated.

If the sensor output is not higher than the expected model value, thenat 420, it may be determined that no soot is present in the exhaust, andit may be inferred that the filter is not degraded at 422. At 424, itmay be determined if the estimated regeneration CO₂ level (or profile)is lower than a threshold. As such, during filter regeneration, anincrease in CO₂ levels (such as, a sudden burst in CO₂ level followingthe initiation of regeneration) may be expected due to the burning offthe stored soot. Thus, at 426, filter regeneration degradation may bedetermined when the estimated regeneration CO₂ level is lower than thethreshold level. That is, it may be determined that the filterregeneration operation is degraded and that the soot stored on thefilter has not been completely removed. Optionally, an engine controllermay further adjust regeneration conditions, for the same and/or asubsequent regeneration operation, responsive to the indication offilter regeneration degradation at 418. For example, in response to thefilter not being degraded but the filter regeneration operation beingdegraded, the exhaust flow rate for a subsequent regeneration may beincreased, the exhaust temperature used for regeneration may be raised,the duration of regeneration may be increased, and/or regeneration maybe initiated at a lower soot load.

In comparison, if the estimated regeneration CO₂ level is below thethreshold, then at 428, it may be determined that the filter is notdegraded and that the filter regeneration operation is also notdegraded.

Thus, during filter storing, an engine controller may electrically heatthe substrate with the heater, estimate a post-filter exhaust CO₂ levelto infer the presence of post filter exhaust PMs with the CO₂ sensor,and control filter regeneration and adjust engine operation based on theestimated exhaust CO₂ level. Similarly, during a filter regenerationcondition, different from the filter storing condition, the controllermay disable the heater and non-electrically heat the substrate using thehot exhaust gas used for filter regeneration. The controller may thenestimate a post-filter exhaust CO₂ level to infer the presence of postfilter exhaust PMs with the CO₂ sensor, and control filter regenerationand adjust engine operation based on the estimated exhaust CO₂ level. Ineither condition, the sensor may also enable the controller to determinefilter degradation based on a CO₂ signature generated from oxidizedpost-filter PMs.

While the depicted routines illustrate adjusting engine operation anddetermining filter degradation based on the CO₂ signature, it will beappreciated that the engine controller may be further configured todiagnose CO₂ sensor degradation based on the estimated CO₂ level.Diagnosing CO₂ sensor degradation may include setting a correspondingdiagnostic code.

In one example, during filter storing, CO₂ sensor degradation may bediagnosed by estimating a first CO₂ level with the substrate heater on,estimating a second CO₂ level with the substrate heater off, andindicating CO₂ sensor degradation when a difference between the firstCO₂ level and the second CO₂ level is less than a threshold. Forexample, when it is known that the filter is not degraded, by comparingthe CO₂ level estimated by the CO₂ sensor with the heater on and off,sensor degradation may be identified due to the sensor not detecting thepresence of an anticipated amount of CO₂.

In another example, during filter regeneration, CO₂ sensor degradationmay be diagnosed by determining a first rate of change in the estimatedCO₂ level over at least a duration of the filter regeneration operation,determining a second rate of change in the pressure across the filterover the same duration, and indicating sensor degradation when adifference between the first and second rate of change is greater than athreshold. Alternatively, sensor degradation may be indicated when aratio between the first and second rate of change is greater than athreshold. Thus, for example, when it is known that the filter is notdegraded and the filter is regenerating, by comparing a parameterreflective of filter regeneration (such as a change in pressure, ortemperature, or mass, etc., across the filter) over a duration of theoperation with the change in the output from the sensor over the sameduration, sensor degradation may be identified due to the sensor outputnot changing at a rate proportional (or normalized) to the rate ofchange of the regeneration parameter across the filter.

Now turning to FIG. 5, an alternate example routine 500 is depicted foridentifying particulate filter degradation based on the output of adownstream CO₂ sensor. At 502, it may be confirmed that the engine isrunning. As such, if the engine is not running, the routine may end. At504, engine operating conditions may be estimated, measured and/orinferred. At 506, CO₂ levels may be estimated and/or inferred based onthe estimated engine operating conditions (CO2_model). In one example, aCO₂ model may be used to estimate an expected CO₂ level based on theengine operating conditions. The level estimated by the CO₂ model maytake into account CO₂ levels anticipated from exhaust PMs, exhaust CO₂,unburned and partially burned exhaust hydrocarbons, etc. At 508, themodeled CO₂ level may be compared to the output of the CO₂ sensor. At510, it may be determined whether the CO₂ sensor output is higher thanthe modeled CO₂ level. If no, then at 514, it may be assumed that theparticulate filter is functioning properly. In comparison, if the CO₂sensor output is higher than the modeled CO₂ level, then at 512, it maybe assumed that the particulate filter is degraded and a filterdegradation indicator may be illuminated. The degradation indicator maybe, for example, a light (such as a malfunction indicator light) on thevehicle dashboard, or a message in a message center presented to thedriver.

It will be appreciated that while one or more of the routines of FIGS.2-5 may be illustrated with reference to diesel particulate filters,this is not meant in a limiting sense, and the same routines may besimilarly applied towards the diagnosis of degradation of otherparticulate filters, such as a gasoline particulate filter in a gasolineengine exhaust system.

In this way, an exhaust CO₂ sensor may be used to determine exhaust CO₂levels as well as to accurately infer an exhaust PM level. Additionally,based on the estimated CO₂ level during filter regenerating andnon-regenerating conditions, filter diagnostics may be performed. Basedon differences between the estimated level and an expected level, engineoperation and/or filter regeneration operation may be further adjusted.By using higher resolution CO₂ sensors, filter diagnostics may beperformed with higher precision.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1-21. (canceled)
 22. A method for controlling an engine with aparticulate filter, comprising, exhausting particulate matter from theparticulate filter to a downstream substrate; controlling temperature ofthe substrate via a heater, exhausted particulate matter being oxidizedon the substrate upstream of a CO₂ sensor; indicating filter degradationbased on a CO₂ signature of the oxidized, post-filter exhaustparticulate matter indicated by the CO₂ sensor; and controlling filterregeneration in response to the CO₂ signature.
 23. The method of claim22, wherein the CO₂ signature is based on a CO₂ level estimated by theCO₂ sensor positioned downstream of the substrate, the method furthercomprising determining an amount of exhausted PMs from the particulatefilter based on the CO₂ level.
 24. The method of claim 23, whereinoxidizing exhaust PMs on the substrate includes operating the substrateheater to heat the substrate, and oxidizing exhaust PMs on the heatedsubstrate using oxygen from exhaust gas downstream of the particulatefilter.
 25. The method of claim 23, wherein the CO₂ signature includesan estimated CO₂ level that is generated during a filter storingoperation.
 26. The method of claim 25, wherein controlling filterregeneration includes initiating filter regeneration in response to theCO₂ level being higher than a threshold during filter storing.
 27. Themethod of claim 25, wherein the indication includes, indicating filterdegradation when the CO₂ level is greater than a threshold.
 28. Themethod of claim 25, wherein the indication includes, indicatingdegradation when the estimated CO₂ level is greater than an expected CO₂level, the expected CO₂ level determined based on engine operatingconditions.
 29. A method of operating a turbocharged engine exhaustsystem including a particulate filter and a downstream electricallyheated substrate, comprising, during substrate non-heating, adjusting anengine boost amount based on an exhaust CO₂ level estimated by a sensor;and during substrate heating, oxidizing post-filter particulate matters(PMs) on the substrate, and initiating particulate filter regenerationbased on a CO₂ signature from the sensor indicative of PM levelsdownstream of the particulate filter
 30. The method of claim 29, whereinthe engine is a gasoline engine.
 31. The method of claim 30, wherein thesensor is a CO2 sensor, and the substrate is upstream of the CO₂ sensor,and wherein during the non-heating condition, the filter is storing andthe substrate is not oxidizing and during the heating condition, thefilter is storing and the substrate is oxidizing.
 32. The method ofclaim 31, wherein the exhaust PMs are oxidized on the heated substrateusing oxygen from exhaust gas.
 33. The method of claim 30, whereininitiating filter regeneration includes initiating filter regenerationwhen the estimated CO₂ level of oxidized, post-filter exhaust PMs isgreater than a threshold.
 34. The method of claim 33, furthercomprising, indicating filter degradation based on the CO₂ signatureindicating a level of oxidized, post-filter exhaust PMs.
 35. The methodof claim 34, wherein the indication includes determining an expected CO₂level based on engine operating conditions, comparing the expected CO₂level to the estimated CO₂ level, and indicating filter degradation whenthe estimated CO₂ level is greater than the expected CO₂ level, whereinindicating filter degradation includes setting a diagnostic code.
 36. Anengine exhaust system, comprising: a particulate filter; a turbocharger;a CO₂ sensor coupled downstream of the filter; a substrate positionedupstream of the CO₂ sensor and downstream of the particulate filter; anda controller having instructions thereon to, during filter storing, heatthe substrate; estimate a CO₂ level corresponding to exhaust PMsoxidized on the heated substrate; initiate filter regeneration based onthe estimated CO₂ level; determine filter degradation based on theestimated CO₂ level; and adjust engine boost amount based on the CO₂sensor.
 37. The system of claim 36, wherein heating the substrateincludes, turning on a substrate heater when an exhaust gas temperatureis below a threshold to electrically heat the substrate; and turning offthe substrate heater when the exhaust gas temperature is above thethreshold to non-electrically heat the substrate using heated exhaustgas.